One method magnetic tape devices utilize to maximize capacity is to maximize the number of parallel tracks on the tape. The typical way of maximizing the number of tracks is to employ servo systems which provide track following and allow the tracks to be spaced very closely. Even so called "low end" tape devices are now employing track following to maximize the number of tracks.
An example of track following servoing is the provision of groups of prerecorded parallel longitudinal servo tracks that lie between groups of longitudinal data tracks, so that one or more servo heads may read the servo information and an accompanying track following servo will adjust the lateral position of the head or of the tape to maintain the servo heads centered over the corresponding servo tracks. The servo heads are spaced a defined distance from the data heads, so that centering the servo heads results in the data heads being centered over the data tracks. The defined distance is maintained for all tape drives in a particular family allowing exchange of tape media between tape drives in the same or compatible families.
An example of a track following servo system particularly adapted to tape comprises that of the Albrecht, et al. 08-270207 application. The servo patterns are comprised of magnetic flux transitions recorded in continuous lengths at non-parallel angles, such that the timing of the servo transitions read from the servo pattern at any point on the pattern varies continuously as the head is moved across the width of the servo pattern. For example, the pattern may comprise straight transitions essentially perpendicular to the length of the track alternating with sloped or slanted transitions, each comprising a pair of transitions. Thus, the relative timing of transitions read by a servo read head varies linearly depending on the lateral position of the head. Speed invariance is provided by utilizing a group of interlaced pairs of transitions and determining the ratio of two timing intervals, the interval between two like transitions compared to the interval between two dissimilar transitions. Synchronization of the decoder to the servo pattern may be accomplished by having two separate groups of pairs of transitions, each group having a different number of pairs of transitions. Thus, the position in the set of groups is readily determined by knowing the number of pairs of transitions in the present group.
Although the determination of the lateral position of a head with respect to the width of a tape may be readily accomplished by such servo systems, there is no good means of determining of the longitudinal position of a tape. Rough estimates of longitudinal position of a tape may be made by counting the number of rotations of an idle guide wheel or of a motor or reel, for example by having an index mark on the wheel, etc. More accurate longitudinal position information relative to data records may be based on detection of the data records themselves. There are a number of problems with these approaches. One is a tape cartridge which was ejected without being rewound so that the count of index marks may be meaningless. Another is locating a record based on an index table of its position by reading records continuously until the correct record number is found. This is a major problem if one of the records is damaged, or if write skipping is allowed. With write skipping, multiple copies of a record are allowed, or subsets of a record are allowed, if the first copy is bad. Any error recovery procedure is now complicated by uncertainty as to which copy of the record is being read.
Another example is to use a fineline tachometer used to give a large number of positions per revolution of a motor or reel, perhaps in the hundreds. However, the fineline tachometer adds to the cost of the drive, making it unusable for low end tape drives. It also occupies considerable space, increasing the reel motor spindle height and making a low height form factor more difficult to achieve and preventing the use of low cost off-the-shelf motors.